Technical Field
The present invention relates to a crystal substrate, an ultraviolet (UV) light-emitting device, and manufacturing methods therefor. More specifically, the present invention relates to a crystal substrate suitable for fabricating devices using a group-III nitride semiconductor crystal, a UV light-emitting device using group-III nitride semiconductor crystal, and manufacturing methods therefor.
Background
Functional devices such as blue light emitting diodes (LEDs) and blue laser diodes (LDs) have been reduced into practice by adopting a crystal of group-III nitride semiconductor, a type of compound semiconductors. Radiation of shorter wavelength than blue light has been sought in the field of solid state light sources, ultraviolet light-emitting diodes (UVLEDs) have been developed accordingly. In particular, ultraviolet (UV) light in a deep UV wavelength range, or a wavelength range below 350 nm, are considered useful in many applications. Since light of 260-280 nm range in the deep UV, a part of a UV-C wavelength range, have vast applications including sterilization, water purification, and medical applications, thus LEDs for the UV-C wavelength range, or deep ultraviolet LEDs (DUVLEDs) have been developed. Typical DUVLED uses a sapphire crystal plate or an AlN single crystal substrate and is made to form a layered structure of gallium-aluminum nitride (AlGaN) system semiconductor, whose main elements are aluminum (Al), gallium (Ga), and nitrogen (N). Improvement of output power of DUVLEDs is in progress; DUVLEDs with UV output level of ˜10 mW have been manufactured to date.
Technological challenges for such UVLEDs include improvement of emission efficiency. The improvement has been eagerly sought by, for example, fabricating quantum well structures into an emission layer, or by adopting a non-polar plane, which may enable increase of overlap of wavefunctions between electrons and holes. Concerning ease of the crystal growth, (0001) plane growth or c-plane growth is advantageous; however, it produces polarization along a thickness direction caused by difference in electronegativity. When such polarization is present, wavefunctions for electrons and holes begin to separate from each other as if they feel repulsion along a thickness direction due to their opposite electric polarity, and it results in degradation of recombination probability in the emission. In addition, when an electron blocking layer is fabricated, tunneling probability becomes high due to the polarization, which may lead to increase of overflow of electrons. Moreover, since a gradation of an electric field is susceptible to a carrier density, an emission wavelength may shift by variation of a driving electric current.
When manufacturing a DUVLED by using a non-polar plane as an example of manufacturing devices of group-III nitride semiconductor, it is favorable that aluminum nitride (AlN) is formed before the growth, as AlN may exert a compression strain in the final device. Therefore, it is advantageous if a non-polar AlN buffer layer is disposed on a readily available sapphire crystal plate. Typical types of a non-polar AlN buffer layer that can be adopted in this application are AlN layers of (1-100) plane and (11-20) plane, which may be referred to as an “m-AlN layer” and “a-AlN layer” respectively in this application. Among these types, the m-AlN layer is so difficult to manufacture that only homo epitaxial growth on an expensive AlN wafer has been reported so far. In contrast, the a-AlN layer can be provided with fairly good lattice matching capability when it is disposed on (1-102) plane sapphire crystal plate, or “r-sapphire crystal plate”.